The present invention relates to a multiplexing liquid crystal display device.
Liquid crystal display devices have been put to popular use as display devices for electronic instruments. Information to be displayed on a conventional liquid crystal display device has been mostly in the form of comparatively simple figures consisting of a small number of picture elements. The conventional liquid crystal display device displays no more than the comparatively simple figures because (1) it is difficult technically to make the display electrodes very complex and (2) it is difficult to have an electrical connection between a liquid crystal display panel and a driving circuit for the liquid crystal panel if the number of terminal electrodes is large. As a means to display complicated figures, a multiplexing driving method, which overcomes the above mentioned disadvantages, has been invented for use in conventional liquid crystal display devices. However, several difficult problems have arisen in applying the multiplexing driving method to the liquid crystal display and these problems have not been solved yet.
One of the problems is that the deterioration of the liquid crystal display element is accelerated if the elements are not driven by AC voltage.
If select display voltages and nonselect display voltage applied to scanned display electrodes of the liquid crystal display panel are respectively .phi..sub.Y and .phi..sub.Y, and if the waveforms of the select display voltages and the nonselect display voltages applied to signal display electrodes of the liquid crystal display panel are respectively .phi..sub.X and .phi..sub.X, the states of the voltage waveforms of the liquid crystal display panel are represented by the following equations: EQU .phi..sub.1 =.phi..sub.Y -.phi..sub.X (a) EQU .phi..sub.2 =.phi..sub.Y -.phi..sub.X (b) EQU .phi..sub.3 =.phi..sub.Y -.phi..sub.X (c) EQU .phi..sub.4 =.phi..sub.Y -.phi..sub.X (d)
If the total number of the scanned display electrodes is M and the number of the select display picture elements and the nonselect display picture elements of some signal display element are respectively m.sub.1 and m.sub.2, M=m.sub.1 +m.sub.2.
Accordingly the voltages having waveforms .phi..sub.1, .phi..sub.3 and .phi..sub.4 are applied to the select display picture elements respectively once, (m.sub.1 -1) times and (M-m.sub.1) times during one frame interval. While the voltages having waveforms .phi..sub.2, .phi..sub.3 and .phi..sub.4 are applied to the nonselect display picture elements respectively once, m.sub.1 times and (M-m.sub.1 -1) times during one frame interval. Accordingly some AC voltage is also applied to the nonselect display picture elements generally. And if a threshold voltage of the liquid crystal display panel is lower than the root mean square voltage (referred to as the rms voltage hereafter) of the voltage waveform applied to the nonselect display picture elements, the picture elements that are intended to be the nonselect picture elements are displayed on the liquid crystal display panel and the so called cross talk is present.
Generally, it is necessary to prevent dispersion of the cross talk density, if cross talk should be present, by fixing the rms value of the voltage waveform applied to the nonselect display picture elements under any conditions for display. In order to prevent the dispersion of the cross-talk, .vertline..phi..sub.3 .vertline., the rms value of .phi..sub.3, and .vertline..phi..sub.4 .vertline., the rms value of .phi..sub.4, must be the same. Accordingly, as for the select display picture elements, .phi..sub.1 is applied once in M times and the voltage equivalent to .phi..sub.3 is applied the remaining (M-1) times during one frame interval. On the other hand, as for the nonselect picture elements, .phi..sub.2 is applied once in M times and the voltage equivalent to .phi..sub.3 is applied the remaining (M-1) times during the same one flame interval. Therefore, the voltage equivalent to .phi..sub.3 is applied to both the select display picture elements and the nonselect display picture elements for (M-1) times in M times. And the select display picture elements and the nonselect display picture elements are distinguished by the remaining one voltage .phi..sub.1 or .phi..sub.2. Namely, the larger M becomes, the smaller the difference of the rms value between the voltage applied to the select display picture elements and the voltage applied to the nonselect display picture elements becomes. Consequently, it is desirable that the total number of scanned display electrodes M is at a minimum.
However, if the number of the scanned display electrodes is reduced, the layout of the display patterns to be displayed on it becomes difficult to realize a liquid crystal plane is limited and the variable display.
As a means of realizing a variable display with a small number of scanned display electrodes, a multiple matrix liquid crystal panel and a multiplexing liquid crystal display panel provided with electrodes shown in FIG. 1 have been proposed.
The multiplexing liquid crystal display panel in FIG. 1, conventionally well known, comprises a plurality of U-shaped scanned display electrodes 11 and pairs of signal display electrodes 12, i.e., the upper and lower portions 12a and 12b. By driving the signal display electrodes 12a and 12b separately, the multiplexing liquid crystal display panel can be operated to display information as if the number of the scanned display electrodes is doubled. On driving the multiplexing liquid crystal display panel, the scanned select display voltage is applied to the scanned display electrodes. In case where one character or letter is comprised of picture elements 5 columns and 7 rows (referred to as 5.times.7 characters hereafter) and is displayed on the multiplexing liquid crystal display panel shown in FIG. 1 (referred to as the "U-shaped" electrode panel hereafter), an interval between electrode portions 21a and 21b of a "U-shaped" scanned display electrode 21 at an inner portion is wide enough so as to be distinguished from an interval between the electrode portion 21a and the neighboring electrode, e.g., an interval between the electrode portions 21a and 22a. By leaving intervals between the characters displayed on the upper and lower portions sufficiently large, the display of the different characters is easily recognized. In this arrangement of the scanned display electrodes, a very effective display is possible with regard to the 5.times.7 characters displayed on the upper and lower portions of the display panel as shown in the display condition in FIG. 3.
However, the scanned display electrodes arrangement is disadvantageous with regard to display of other characters and drawings formats. For instance, in case a letter composed on the picture elements of 7 columns and 9 rows is displayed using the "U-shaped" electrode panel, the characters are divided in two and the two parts of the characters are displayed separately on the upper and lower portions of the "U-shaped" electrode panel, and the interval between the electrode portions 21a and 21b exists with the characters divided as shown by an example of such a display condition shown in FIG. 4, and as a result the display is unsightly.
On the other hand, as for the display condition in which cross-talk is present, the picture elements to be essentially in the nonselect display condition are displayed slightly, as indicated by oblique lines in FIGS. 3 and 4. In this display condition a wide belt appears as an interval through the cross talk on the panel, and as a result the display is exceedingly unsightly.